A. Field of the Invention
The present invention relates to machinery for use in drilling and operating oil and gas wells. More particularly, the invention relates to a particular type of mechanism for preventing pressurized liquids or gases from blowing out and upwards through a well hole in an uncontrolled manner. Such mechanisms are referred to in the industry as ram blowout preventers. Specifically, the invention relates to an improved automatic locking actuator piston for use in ram blowout preventers.
B. Discussion of Background Art
In drilling for natural gas or liquid petroleum, a drill string consisting of many lengths of threaded pipes which are screwed together, and terminated at the lower end of the string by a drill bit cutting head, is used to bore through rock and soil. The drill bit head has a larger diameter than the pipes comprising the drill string above the head. A rotary engine attached to the upper end of the drill string transmits a rotary boring motion to the drill bit head.
During the drilling operation, a specially formulated mud is introduced into an opening in an upper drill pipe. This mud, which generally is of a type having a high specific gravity, flows downwards through the hollow bores of the pipes in the drill string and out through small holes or jets in the drill bit head. Since the drill bit head has a larger diameter than the drill string above it, an elongated annular space is created between the outer walls of the drill string components and the walls of the drilled hole during the drilling process. This annular space permits the mud to flow upwards to the surface. Mud flowing upwards carries drill cuttings, primarily rock chips, to the surface. The mud also lubricates the rotating drill string, and provides a downward hydrostatic pressure which counteracts fluid pressure which might be encountered when the drill string enters subsurface gas pockets, or liquids under pressure.
In normal oil well drilling operations, it is not uncommon to encounter subsurface gas pockets whose pressure is greater than could be counter-balanced solely by the hydrostatic pressure of the elongated annular column of drilling mud. To prevent the explosive and potentially dangerous and expensive release of gas and/or liquid under pressure upwards out through the drilling hole, machines called blowout preventers are used. Blowout preventers are mounted in a pipe casing surrounding a drill hole, near the upper end of the hole.
Typical blowout preventers have resilient sealing means which can be made to tightly grip the outer circumferential surfaces of various diameter drill string components, preventing pressure from subterranean gas pockets from blowing out material upwards along the drill string. Usually, the resilient sealing means of a blowout preventer is designed to move a plurality of sealing elements into forcible contact with one another, when all components of a drill string are removed from the casing. This permits complete shutoff of the well, even with all drill string components removed. Most oil well blowout preventers are remotely operable, typically, by a hydraulic pressure source near the drill hole opening, the pressure source being coupled to a hydraulic actuator cylinder in the blowout preventer via hydraulic lines.
Ram blowout preventers (BOP's) utilize a pair of opposed semicircular blocks driven radially inwards by opposed transverse hydraulic rams towards the periphery of a tubular oil well component extending through a longitudinally disposed bore in the BOP. Each of the semicircular ram blocks contains a semicircular sealing element which has formed in its flat diametrical face a coaxial, semicircular groove adapted to conformally engage the periphery of a tubular component within the bore. The faces of the ram sealing elements usually include resilient elements to seal against one another and with the periphery of the oil well component. The purpose of the resilient elements is to form an effective pressure-tight seal against down-hole pressures, which may be as high as 15,000 psi. The resilience of the sealing elements provide a compressibility which accommodates a relatively small range of possible variation in the diameter of tubular oil well components on which the rams are intended to seal against.
Radial motion of ram sealing elements of ram BOPs is usually effected by diametrically opposed hydraulic piston actuators in opposed hydraulic cylinders located on either side of the BOP. A design requirement for most ram BOPs, particularly those that are used in offshore operations, is that they be fail-safe. Thus, the ram elements must remain in sealing contact with the periphery of tubular well components even if hydraulic pressure fails after actuation of the BOP. To fulfill this fail-safe requirement, most ram BOPs utilize a locking piston which includes locking lugs or "dogs" which move radially outward at the end of a piston stroke. The lugs move radially into an annular groove provided in the cylindrical wall of the hydraulic cylinder, near the front end of the cylinder closest to the ram sealing element. Rearward motion of the ram actuator piston is prevented by abutting contact of the rear surfaces of the locking lugs with a locking shoulder forming the rear transverse wall of the annular groove. Unlocking of the lugs is effected by application of hydraulic pressure to the front side of the piston. This reverse pressure also moves the piston and ram sealing elements to a rearward, unlocked position.
Typical ram blowout preventers having an automatic locking capability have a smaller diameter piston, with a rounded front edge, longitudinally slidably contained within a larger diameter, hollow main hydraulic ram actuator piston. Locking lugs are radially slidably contained within radially disposed slots provided at regular circumferential intervals through the outer cylindrical wall of the main piston. The inner radial ends of the locking lugs are slidably engaged by the rounded front edge of the smaller piston. Closing hydraulic pressure on the rear surface of the large and small pistons moves both pistons forward within the cylinder. Forward motion of the main piston is halted when the sealing element on the end of the ram shaft attached to that piston abuts a fixed object. At this point, the smaller piston moves forward within the main piston until its rounded front face contacts the inner ends of the lugs and moves them radially outwards into locking engagement with the annular groove provided in the forward end of the cylinder wall.
Opening hydraulic pressure applied to the front face of the large and small pistons moves the inner piston rearward with respect to the larger piston, allowing the locking lugs to move radially inwards. Disengagement of the locking lugs from the locking groove permits the main, larger piston to move rearward. Rearward movement of the larger main piston and forward extending ram shaft attached to the main piston pulls the attached ram block and ram sealing element away from the drill string component.
The principles of operation of a ram blowout preventer, as described above are simple and readily understood. Thus, it might be concluded that ram blowout preventers would be simple to construct and operate. In fact, the structure and operation of existing ram blowout preventers causes substantial operational difficulties when they are used in typical oil drilling operations, for reasons which will now be described.
Resilient sealing material, often referred to as a ram rubber, in the face of the ram block sealing elements wears thin after a number of sealing and unsealing cycles. Reduction in thickness of the ram rubber results in an ineffective seal, unless the ram block is moved forward slightly from its initially set, locked position. Re-adjustment requires turning the threaded ram shaft within the engaging threads in the ram piston to move the ram shaft and ram block forward slightly, to compensate for wear of the sealing element. This adjustment must result in a precisely controlled locking position, in which the ram rubber must seat sufficiently tightly against the wall of a drill string component to resist blowout pressures as high as 15,000 psi, yet not damage the drill string component. Usually, the skill required to make this adjustment, which may be as small as a fraction of a screw thread, necessitates flying out a factory-trained specialist to the drilling site, at a very considerable expense to the drill operator.
Ram blowout preventers of the type described above, in which a resilient sealing element is used to seal on a particular diameter drill string component, belong to a particular class of ram blowout preventers referred to as pipe ram blowout preventers or pipe rams. Another type of ram blowout preventer, referred to as a shear ram, shears off a drill pipe and effects a seal between the two severed halves. This type of blowout preventer also requires a very precise adjustment of its longitudinal locking position.
A third type of ram blowout preventer, referred to as a variable bore ram blowout preventer, is used to seal drill string components having a diameter which may vary over a substantially broad range. The structure and operation of variable bore blowout preventers is described in our disclosure of a novel variable bore ram rubber in our patent application entitled, "Variable Bore Ram Rubber" Granger, Beard and Sveen, filed Apr. 29, 1988, Ser. No. 188,267, now U.S. Pat. No. 4,930,745, issued June 5, 1990.
Each of the three types of ram blowout preventers described above requires a different adjustment procedure which must be performed by an experienced, highly skilled individual. The variable bore ram blowout preventer probably requires the most critical and demanding adjustment. For example, a typical variable bore ram blowout preventer is designed to accommodate drill string components in the diameter range of 31/2 inches to 5 inches. Usually, if the locking position of the ram piston is adjusted so that the resilient ram rubber forms an effective seal against a 31/2 inch pipe, the ram blowout preventer will not produce an effective seal against the surface of a 5 inch pipe, and vice versa. To counter this problem, ram blowout preventers have been manufactured which use a two-stage locking piston. However, these devices are extremely complicated and difficult to adjust, and have therefore not been widely used.
In view of the difficulties associated with the operation of ram blowout preventers described above, the self-adjusting automatic locking piston for ram blowout preventers according to the present invention was conceived of.